mcl is a MUD client running in a Virtual Console
under Linux or using standard TTY routines under
any other UNIX. Embedded Perl or Python offer very
high tweakability. Other features include huge
scrollback, a stable input line with the MUD
prompt, Perl regexps for actions and substitutions.

Momoko is a generic, object-oriented, modular, extensible environment for creating multi-user environments. It is suitable for writing MUDs, servers, and community-based Web sites. It is written in pure Java and supports Java and Python as internal scripting languages. Momoko provides facilities for organizing objects into tree-based hierarchies, orthogonal persistence, a transparent distributed objects system, remote access to source code and user files via WebDAV, a telnet-enabled shell for manipulating the object database, and an integrated build process which can be triggered remotely.

Mordor is a MUD (Multi-User Dungeon), developed in ANSI C on Linux, that will run on most UNIX and Microsoft systems. It includes source code, Linux binaries, and a large stock database for quick and easy set up on an Intel-based system.